Information for clinicians
9 Molecular genetics
Table 9.1: Molecular genetics of adult Refsum's disease
|Gene symbol||Chromosomal locus||Protein name|
|PEX7||6q22-q24||Peroxisomal targeting signal 2 receptor|
|PHYH||10pter-p11.2||Phytanoyl-CoA dioxygenase, peroxisomal|
Table 9.2: OMIM Entries for adult Refsum's disease
|OMIM reference number||Description of OMIM entry|
|601757||PEROXISOME BIOGENESIS FACTOR 7; PEX7|
|602026||PHYTANOYL-CoA HYDROXYLASE; PHYH|
Table 9.3: Genomic databases for adult Refsum's disease
|Gene symbol||Entrez Gene||HGMD||GeneCards||GDB||GenAtlas|
Mutations in PHYH and PEX7 are known to cause adult Refsum's disease by interfering with the alpha-oxidation of phytanic acid.
(Section 6 - Biochemistry - gives more information on alpha-oxidation of phytanic acid.)
9b(i) Normal allelic variants
Jansen et al (2004) have described one sequence variant (c.636A>G) with an incidence of around 10% in 93 control individuals, which causes no amino acid change.
Sequence analysis of the PHYH gene has now been performed in 60 unrelated families and has revealed 31 different variants of which 13 (41.9%) are unique to one patient or family. (Click here to review table of results (Table 9.4).) Of these 31 variants:
- Nineteen (61.3%) are missense.
- Four (12.9%) are deletions. All deletions are found in the first half of the coding sequence and all cause a frame shift.
- Two are insertions (6.5%). One causes a frame shift; the other, a three nucleotide insertion, causes the insertion of a single amino acid into the PhyH protein.
- Six (19.4%) are splice site. Two splice site variants are located in IVS2 (c.135-2A>G and c.135-1G>C), which leads to skipping of exon 3, consisting of 111 nucleotides. This causes an in-frame deletion of 37 internal amino acids, and an altered protein (p.Y46-R82del), which, when heterozygously expressed in S. cerevisiae, is clearly detectable by Western blot analysis, but completely lacks enzymatic activity [Jansen et al 2000]. Four splice site variants, one in IVS5 (c.497-2A>G) and three in IVS6 (c.678+2T.G, c.678+5G>T, and c.679-1G>T), cause skipping of exon 6, which results in a frameshift and a premature stop codon (p.A166fsX3).
- One (3.2%) of the missense variants is a non-disease-causing variant which is also present in about 10% of a control population.
9b(iii) Abnormal gene product
The impact of any mutation in the phytanoyl-CoA hydroxylase gene can be assessed by evaluating the consequences of certain mutations on the stability and the catalytic activity of the hydroxylase upon expression. Interestingly, 15 of the 17 missense mutations identified in the phytanoyl-CoA hydroxylase are located in exon 6 and 7. Structure function analysis has demonstrated that these exons code in part for the conserved beta-barrel core, which suggests that this structurally important element in the protein is susceptible to changes that directly cause loss of enzymatic activity. Click here for diagram of crystal structures.
The effect of only very few of the missense mutations has been tested by expressing the mutant proteins and testing protein stability and enzyme activity. Mukherji et al (2001) have studied a few mutants including the p.H175R, p.Q176K, and p.D177G mutants. The amino acid triad 175-177 (HQD) forms the iron binding motif and mutations in any of these amino acids have been shown to cause a fully dysfunctional enzyme since the hydroxylase is completely dependent upon Fe2+ for the conversion of phytanoyl-CoA to 2-hydroxyphytanoyl-CoA. Click here for diagrams of mutations related to the enzyme active site.
Two missense mutations, including p.G204S and p.N269H cause the peculiar effect of uncoupling the hydroxylation of phytanoyl-CoA from the conversion of 2-oxoglutarate into succinate and CO2. As demonstrated in expression studies, Mukherji et al (2001) found that no phytanoyl-CoA was hydroxylated while decarboxylation of 2-oxoglutarate to succinate and CO2 still took place, although at a much-reduced rate. This uncoupling is also observed in case of the p.Q176K substitution, which is also associated with a change in the iron binding motif.
9c(i) Pathogenic allelic variants
So far, three individuals have been identified with mild mutations in the PEX7 gene.
9c(ii) Abnormal gene product
The abnormal protein results in partially impaired import of peroxisomal proteins including phytanoyl-CoA hydroxylase (see Braverman et al 2002, van den Brink et al 2003b).
Next: Literature cited
Page last updated 26 June 2006